From Wiktionary, the free dictionary
Sino-Vietnamese word from 次男, composed of 次 (“next”) and 男 (“male”).
thứ nam
- (formal, chiefly on wedding invitation cards or at weddings) the second, but not youngest, son
- Coordinate terms: ái nữ, quý nam, quý nữ, thứ nữ, trưởng nam, trưởng nữ, út nam, út nữ